This invention relates to a digital information storage apparatus with means for correcting errors, more particularly to a method of generating dropout flags to enhance the effectiveness of an error-correcting code.
Correction of errors in digital information read from an information storage apparatus in enabled by storing the information in an encoded form, consisting of groups of codewords in a format such as shown in FIG. 4, this diagram being equivalent to one presented on page 205 of Nikkei Electronics No. 21, November 1983 but with a different code configuration. The format in FIG. 4 begins with a preamble 1, the purpose of which is to simplify clock signal generation when the information is read. The preamble consists of a highly periodic pattern, such as repetitions of the pattern "100" Following the preamble 1 are synchronization fields 2 and data fields. Each of the data fields consists of a distinctive pattern used for synchronization of the data in the data fields. The data fields comprise digital information 3a and check symbols 3b. The synchronization fields 2 are placed periodically throughout the data fields 3a and 3b as well at the beginning of the data. The configuration in FIG. 4 is segmented into 12-byte blocks, with a one-byte synchronization field 2 added to every block. In the information storage apparatus these fields are recorded in the following order: the preamble 1, a synchronization fields 2, then data bytes D1, D131, D261, D391, D2, D132, . . .
The data field is divided into four codewords extending in the horizontal direction in the drawing. One codeword P comprises 130 bytes of digital information 3a and 16 bytes of check symbols 3b. A Reed-Solomon encoding scheme with Hamming distance 17 (146, 130, 17) is employed. The error correcting capability of a Reed-Solomon code can be described in terms of three parameters: E, the number of errors at unknown locations; F, the number of errors at known locations; and D, the Hamming distance. Errors in the encoded information can be corrected whenever the condition in Eq. (1) is satisfied: EQU F+2E&lt;D (1)
usually, the location of the errors is unknown. Condition (1) then becomes: EQU 2E&lt;D (2)
In the present case which D=17, Eq. (2) implies that errors can be corrected at a maximum of 8 unknown locations.
Methods of error correction by (146, 130, 17) Reed-Solomon codes will not be described in detail here, but information can be found in U.S. Pat. No. 4,162,480 and in research publication PRL73-77 (January 1974) of the Institute of Electronics and Communication Engineers of Japan.
Consider now the effects of a defect in the storage medium, such as a defect contaminating the bytes marked X in FIG. 5, on the reproduction of information recorded in the format just described. The error A preventing the correct reproduction of data over an extended interval, leads to bit slip in the clock signals at the positions marked with triangles. This causes loss of data synchronization; that is, number of clock pulses does not match the quantity of data, causing data errors until the next synchronization field 2d is detected correctly. As as result, all the data marked with triangles are read incorrectly. Short errors such as the one at B do not cause bit slip. In the example shown in FIG. 5, error A causes a 7-byte error in each codeword P Unless means are provided for location the error A, equation (2) implies that this error uses up almost all of the code's error-correcting capability, leaving enough to correct an error in only one additional byte.
Bit slip thus poses a serious problem in the prior art of error correction, because it causes a large number of errors that must be corrected without accurate information as to their location, placing a considerable load on the error-correcting capability of the error-correcting code.